Many fluoropolymer materials, such as polytetrafluoroethylene (PTFE), are thermoplastic polymers. That is, they have the property of softening when heated and of hardening again when cooled. PTFE is generally produced in the form of white powder referred to as resin. It has a higher crystalline melting point (327.degree. C.) and higher viscosity than other thermoplastic polymers, which makes it difficult to fabricate in the same manner as other plastics.
PTFE is a long chain polymer composed of CF.sub.2 groups. The chain length determines molecular weight, while chain orientation dictates crystallinity. The molecular weight and crystallinity of a given resin prior to sintering are controlled by the polymerization process.
Currently, three different types of PTFE resins are available which are formed from two different polymerization processes. The three resins are granular polymer, aqueous dispersions, and coagulated dispersion products.
In the coagulated dispersion of PTFE resin, small diameter (0.1-0.2 micrometer) particles are coagulated under controlled conditions to yield agglomerates ranging in size from 400 to 500 micrometers in diameter. The morphological structure of these agglomerates can be considered as long chains of PTFE that are intermingled in a tangled network.
A known method of forming articles from fluoropolymer resins, such as PTFE, is to blend a resin with an organic lubricant and compress it under relatively low pressure into a preformed billet. Using a ram type extruder, the billet is then extruded through a die in a desired cross-section. Next, the lubricant is removed from the extruded billet by drying or other extraction method. The dried extruded material (extrudate), is then rapidly stretched and/or expanded at elevated temperatures. In the case of PTFE, this results in the material taking on a microstructure characterized by elongated nodes interconnected by fibrils. Typically, the nodes are oriented with their elongated axis perpendicular to the direction of stretch.
After stretching, the porous extrudate is sintered by heating it to a temperature above its crystalline melting point while it is maintained in its stretched condition. This can be considered as an amorphous locking process for permanently "locking-in" the microstructure in its expanded or stretched configuration.
It has been found that the effect caused by stretching PTFE is dependent on extrudate strength, stretch temperature, and stretch rate. According to U.S. Pat. No. 3,953,566 of W. L. Gore, products expanded at high rates of stretch have a more homogenous structure and possess much greater strength. Extrudate strength is more generally a function of the molecular weight and degree of crystallinity of the starting resin and extrusion conditions such as extrusion pressure, lubricant level, and reduction ratio. These parameters also control the degree of alignment that results from extrusion. The degree of alignment, in turn, affects one's ability to homogeneously stretch the extrudate.
Molecular weight and crystallinity affect the stretch characteristics, sinter profile and ultimately the final properties of the processed material. For the initial stages of fabrication, most PTFE fine powders used for ram extrusion and expansion processing are highly crystalline (&gt;90%). as determined by IR spectroscopy, but their molecular weights may differ.
Low molecular weight materials tend to crystallize quickly and become highly crystalline and very brittle. In addition, the intermolecular forces between difluoromethylene groups are very low. Thus, in order to achieve adequate strength, one needs either very high molecular weight, highly crystalline material or one needs some way to disrupt the crystalline order. With a homopolymer, the best way to inhibit crystallization is to increase the viscosity of the molten material to very high values by selecting a polymer with very high molecular weight. In fact, PTFE coagulated dispersion resins that have very high molecular weights with molecular weight distributions have been developed for expanded PTFE processes.
In line with these considerations, the primary function of the "sintering" step is to heat the polymer above its crystalline melt point so that it can be reformed upon cooling to a low enough crystalline content to achieve the sort of mechanical properties required for the current application. To maintain a low crystalline content in the final product, the melt viscosity, corresponding to the molecular weight of the polymer, must be very high.
Most known methods for processing PTFE describe unilateral stretching techniques and stress the importance of stretching the fluoropolymer at rapid rates. For example, U.S. Pat. Nos. 3,953,566 and 4,187,390 issued to Gore state that while there is a maximum rate of expansion beyond which fracture of the material occurs, the minimum rate of expansion is of much more practical significance. Indeed, the patents state that at high temperatures within the preferred range for stretching (35.degree. C.-327.degree. C.) only the lower limit of expansion rate has been detected. The patents estimate this rate to be ten percent of the initial length of the starting material per second. The patents go on to note that the lower limit of expansion rates interact with temperature in a roughly logarithmic fashion so that at higher temperatures within the preferred stretching range, higher minimum expansion rates are required.
U.S. Pat. No. 4,973,609 to Browne describes another method for producing porous PTFE products by stretching at a rate of 10% per second. The patent also states that a differential structure is obtained by using an alloy of two different fluoropolymer resins which are characterized by significantly different stretch characteristics. The resins have different molecular weights and/or crystallinities. Accordingly, the final physical properties, such as strength, of PTFE articles formed in such a way are affected by the different molecular weights and/or crystallinities of the starting resins.
U.S. Pat. Nos. 4,208,745 and 4,713,070 also describe methods for producing porous PTFE products having a variable structure. The processes utilize a sintering step having a differential sintering profile. That is, one surface of an expanded PTFE article is sintered at a temperature which is higher than the sintering temperature of another surface. This results in fibrils being broken and provides an inherently weak material.